Porous membranes made of thermoplastic resin are widely used as materials for separation, selective permeation and isolation of substances, and the like. The porous membrane is used in battery separators used for lithium secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries and polymer batteries, separators for electric double layer capacitors, various kinds of filters such as reverse osmosis membrane filters, ultrafiltration membranes and microfiltration membranes, moisture-permeable waterproof garments, medical materials and the like. Particularly porous membranes made of polyethylene are suitably used as lithium ion secondary battery separators because the porous membrane made of polyethylene has not only such features that it is excellent in electric insulation, has an ion permeability owing to impregnation of an electrolytic solution and is excellent in resistance to electrolytic solution/oxidation resistance, but also a shutdown effect of suppressing an excessive rise in temperature by interrupting a current at a temperature of about 120 to 150° C. at the time of an unusual rise in temperature of a battery. If a rise in temperature after shutdown continues for some reasons, however, breakage of a membrane may occur at a certain temperature due to a reduction in viscosity of molten polyethylene constituting the membrane and shrinkage of the membrane. If the membrane is left standing under a certain high temperature, breakage of a membrane may occur after elapse of a certain time period due to a reduction in viscosity of molten polyethylene and shrinkage of the membrane. This phenomenon is not limited to polyethylene, and cannot be avoided at a temperature equal to or higher than the melting point of a resin constituting the porous membrane even if any other thermoplastic resin is used.
Particularly, a lithium ion battery separator has a significant influence on battery characteristics, battery productivity and battery safety, and is required to have excellent mechanical characteristics, heat resistance, permeability, dimensional stability, pore blocking characteristics (shutdown characteristics), melt membrane breakage preventing characteristics (meltdown preventing characteristics) and the like. Thus, various studies on improvement of heat resistance have been conducted. Further, to improve battery capacity, it is predicted that the thicknesses of not only electrodes but also separators will be still further reduced to increase the area of those components that can be filled in a container. Deformation in the plane direction more easily occurs as the thicknesses of porous films are increasingly reduced, and therefore a heat-resistant resin layer may be peeled off during processing of a composite porous membrane or a slitting step or a battery assembly step so that it becomes difficult to ensure safety.
To keep up with cost reduction, it is expected that the speed will be increasingly enhanced in the battery assembly step, and we believe that it is also required to diminish troubles such as peeling of the heat-resistant resin layer in such high-speed processing, and for meeting the requirement, a still higher adhesiveness is necessary. Japanese Patent Laid-open Publication No. 2005-281668 discloses a lithium ion secondary battery separator obtained by applying a polyamideimide resin directly to a polyolefin porous membrane having a thickness of 25 μm so that the film thickness of the polyamideimide resin is 1 μm, and immersing the coated membrane in water at 25° C., followed by drying the same.
In a roll coating method, a die coating method, a bar coating method, a blade coating method or the like that is generally used for applying a coating solution to a polyolefin porous membrane as in Japanese Patent Laid-open Publication No. 2005-281668, penetration of a resin component into the polyolefin-based porous membrane cannot be avoided due to the shear force thereof, and a considerable rise in gas permeation resistance and a reduction in shutdown function cannot be avoided. In that method, particularly when the film thickness of the polyolefin-based porous membrane is as small as less than 10 μm, the resin component easily fills in pores, leading to an extreme rise in gas permeation resistance. Further, this method has such a problem that unevenness of the film thickness of the polyolefin-based porous membrane easily causes unevenness of the film thickness of the heat-resistant resin layer, leading to variations in gas permeability resistance.
Japanese Patent Laid-open Publication No. 2001-266942 shows, as an example, an electrolytic solution carrying polymer membrane obtained by immersing a nonwoven fabric formed of aramid fibers having an average thickness of 36 μm in a dope containing a vinylidene fluoride-based copolymer as a heat-resistant resin, and drying the same.
Japanese Patent Laid-open Publication No. 2003-171495 shows, as an example, a composite porous membrane obtained through steps of immersing a polypropylene porous membrane having a film thickness of 25.6 μm in a dope having, as a main component, polyvinylidene fluoride as a heat-resistant resin, followed by solidifying the resultant, washing the same with water, and drying the same.
When a unwoven fabric formed of aramid fibers is immersed in a heat-resistant resin solution as in Japanese Patent Laid-open Publication No. 2001-266942, heat-resistant porous layers are formed within and on both surfaces of the unwoven fabric, and therefore continuous pores within the unwoven fabric are mostly blocked so that not only a considerable rise in gas permeation resistance cannot be avoided but also a shutdown function that affects safety of a separator can not be obtained. Further, the unwoven fabric is not suitable for an increase of the capacity of a battery which will be advanced in future because it is difficult to reduce the thickness thereof as compared to the polyolefin-based porous membrane.
In Japanese Patent Laid-open Publication No. 2003-171495, heat-resistant porous layers are also formed within and on both surfaces of the polypropylene porous membrane so that as in the case of Japanese Patent Laid-open Publication No. 2001-266942, a considerable rise in gas permeation resistance cannot be avoided, and it is difficult to obtain a shutdown function.
Japanese Patent Laid-open Publication No. 2001-23602 discloses a separator having a heat-resistant porous layer formed of para-aramid, which is obtained by, when applying a solution of a para-aramid resin as a heat-resistant resin directly to a porous film made of polyethylene having a thickness of 25 μm, preliminarily impregnating the porous film made of polyethylene with a polar organic solvent that is used in the heat-resistant resin solution for avoiding a considerable rise in gas permeation resistance, and forming the porous film into a clouded membrane in a Temperature & Humidity Chamber set at a temperature of 30° C. and a relative humidity of 65% after applying the heat-resistant resin solution to the porous film, followed by washing and drying the same.
In Japanese Patent Laid-open Publication No. 2001-23602, there is no considerable rise in gas permeation resistance. However, the adhesiveness between the porous film made of polyethylene and the heat-resistant resin is extremely low, and particularly when the thickness of the porous film made of polyethylene is less than 10 μm, deformation easily occurs in the plane direction, and therefore the heat-resistant resin layer may be peeled off in the battery assembly step so that it becomes difficult to ensure safety.
Japanese Patent Laid-open Publication No. 2007-125821 discloses a composite porous membrane obtained by applying a polyamideimide resin solution to a propylene film, passing the coated film an atmosphere at 25° C. and 80% RH for 30 seconds to obtain a semi-gel porous membrane, then laminating on the semi-gel porous membrane a polyethylene porous film having a thickness of 20 μm or 10 μm, and immersing the laminate in an aqueous solution containing N-methyl-2-pyrrolidone (NMP), followed by washing the same with water and drying the same.
In Japanese Patent Laid-open Publication No. 2007-125821, there is no considerable rise in gas permeation resistance. However, the adhesiveness between the porous film made of polyethylene and the heat-resistant resin is extremely low, and particularly when the thickness of the porous film made of polyethylene is less than 10 μm, the heat-resistant resin layer may be peeled off as in Japanese Patent Laid-open Publication No. 2001-23602 so that it becomes difficult to ensure safety.
Thus, in a composite porous membrane including a porous membrane such as a polyolefin-based porous membrane as a base material and a heat-resistant resin layer, the heat-resistant resin layer being laminated on the porous membrane, a rise in gas permeation resistance increases when improving an adhesiveness of the heat-resistant resin layer by penetrating a heat-resistant resin into the porous membrane as a base material, while when penetration of the heat-resistant resin is reduced, a rise in gas permeation resistance can be kept low; however, the adhesiveness of the heat-resistant resin layer decreases so that it becomes difficult to ensure safety for which requirements become more and more severe when considering speed enhancement in the battery assembly step while the thickness of a separator is increasingly reduced. Thus, there has not been a composite porous membrane which can achieve both a high adhesiveness of a heat-resistant resin layer and a small rise in gas permeation resistance so far. Further, it becomes more and more difficult to achieve both a high adhesiveness of a heat-resistant resin layer and a small rise in gas permeation resistance as the film thickness of the porous membrane such as a polyolefin-based porous membrane as a base material is reduced.
It could therefore be helpful to provide a composite porous membrane which can achieve both an excellent adhesiveness of a heat-resistant resin layer and a small rise in gas permeation resistance even in the case of further reducing the thickness of composite porous membranes, for example, battery separators, and is intended to provide a composite porous membrane suitable especially for a battery separator, which is suitable for an increase of the capacity of a battery, and to achieve an excellent ion permeability and high-speed processability in a battery assembly processing step.